Going over this again, this was clearly ret-conned. So, Mccree was at least in his early 20’s when he joined Blackwatch.
Concerning McCree’s Age in Blackwatch
Originally, when Ana was released, we saw this picture within her video:
Originally, it was stated that this photo was taken 20 years ago from “present day” Overwatch. Since McCree and Mercy are currently 37, this would mean they were 17 in this picture. Source 1 (YouTube Video from Hammeh, a great lore analysis Youtuber; I highly recommend following him to look over a lot of lore and speculation points for Overwatch if you’re interested). Source 2 (Look at the whole thread, for context of the Tweets).
However, in recent months, Michael Chu and the storyboard team have mentioned that they have not correctly portrayed the ages of the characters within the pictures of Overwatch’s past (mainly Ana, Jack, Gabe, etc). Source. As such, many pictures, including the one above, have been placed in a realm where they are not /exactly/ cannon, but still pieces of lore.
Thus, people became reasonably confused about McCree’s age. Is he still 17 in this photo, and thus 17 when he was recruited, or taken in, by Blackwatch? Or have they changed their ages since they stated they have not portrayed the character’s ages correctly? Remember, it was stated that McCree had a choice to either go to jail, or join Blackwatch. This might not necessarily mean he started doing military level work when he was 17, but may have gone into some kind of bootcamp, or personal training. We will learn, hopefully, as Overwatch’s story develops!
TL;DR: McCree was 17 when he joined Blackwatch, but due to the Storyboard Team mentioning that they have not portrayed the characters correctly within lore related pictures, McCree’s age has been, understandably, questioned by fans.